Parkinson's disease (PD) is the most common neurodegenerative disorder affecting the motor system, and is characterized clinically by bradykinesia and resting tremor, and pathologically by dopaminergic neurodegeneration and intracellular Lewy body deposition. Ubiquitin carboxyl-terminal-hydrolase LI (UCH- L1) is an abundant neuronal deubiquitinating enzyme, and recently the I93M mutation of UCH-L1 was described in a family with autosomal dominant PD. UCH-L1 is implicated in both the ubiquitin proteasome system (UPS) and chaperone mediated autophagy (CMA) protein degradation pathways. PD-linked I93M mutant UCH-L1 demonstrates significantly reduced deubiquitinating activity and additionally, I93M mutant UCH-L1 overexpression is associated with dopaminergic neurodegeneration and inhibition of the CMA pathway. Mice lacking functional UCH-L1 display axonal degeneration, reduced free monomeric ubiquitin level, and motor and learning deficits. Decreased expression of UCH-L1 is observed in cortex of sporadic PD patients, as well as extensive modification by carbonylation and oxidation at methionine and cysteine residues. Together these observations suggest that UCH-L1 dysfunction is detrimental to neurons, and may contribute to PD pathogenesis. Exposure to environmental toxicants which are linked to increased reactive oxygen species (ROS) generation has been identified as a PD risk factor. The mechanism by which environmental toxicants contribute to PD pathogenesis remains unclear, but one possibility is that increased ROS induce oxidative modification of particular target proteins, such as UCH-L1, leading to pathological alteration of their cellular function. This proposal will test the hypothesis that environmental-toxicant induced oxidative stress causes oxidative modification of UCH-L1, thereby disrupting the role of UCH-L1 in the UPS and CMA degradation pathways, which leads to toxic protein accumulation and dopaminergic cell death. The specific aims are to 1) determine the severity of UCH-L1 oxidative modification in the predominantly affected brain region in PD, the substantia nigra, and to evaluate UCH-L1 as a target of oxidative modification caused by environmental toxicant exposures;2) characterize the effects of oxidative modification of UCH-L1 on enzymatic activity and interactions with CMA machinery;and 3) determine the effects of UCH-L1 oxidative modification on the UPS and CMA protein degradation pathways and cell viability. Understanding the role of UCH-L1 oxidative modification and subsequent alterations of its function on protein degradation and cell viability will contribute to the understanding of PD pathogenesis and to the development of novel therapeutics or treatments for this disease.